Dynamic Spectrum Access

Closely related to our work in the areas of Medium Access Control (MAC) and scheduling are theoretical research challenges in the area of dynamic spectrum access. In dynamic spectrum access networks (also known as cognitive radio networks), Secondary Users can use spectrum white spaces (spectrum holes) that are not used by the Primary Users but must avoid interfering with active Primary Users. Such networks are key enablers to efficient use of the spectrum, and therefore, the FCC recently allowed cognitive radio devices to operate in TV bands white spaces.

Our research activities focus on obtaining fundamental understanding of such systems. For example, we considered exploiting temporarily available white spaces and spreading the transmissions of the Secondary Users over a number of non-contiguous sub-channels. While such methods are highly beneficial in terms of spectrum utilization, excessive fragmentation degrades performance. Thus, there is a need to study these processes so as to determine how to ensure acceptable levels of fragmentation. Hence, we presented experimental and analytical results where our main theoretical result shows that even if fragments can be arbitrarily small, the system does not degrade with time. Namely, the average total number of fragments remains bounded. Moreover, extensive simulation results describe behavior, at times unexpected, of fragmentation under different algorithms.

As another example, we considered a system in which Secondary Users choose to either acquire dedicated spectrum or to use white spaces which belong to Primary Users. The tradeoff incorporated in this decision is between immediate yet costly transmission and free but delayed transmission (a consequence of both the possible appearance of Primary Users and sharing the white spaces with multiple Secondary Users). Employing queueing and game theoretic methods, we considered self-interested Secondary Users, studied the interactions between them, and characterized the equilibrium behavior.